One of the most common arrangements for moving large quantities of heavy material such as overburden from strip mining operations, earth from excavation operations, and other similar material movement, is to use large electric or hydraulic excavators to lift the material into large trucks. Hydraulic excavators come with either front shovel boom arrangements or backhoe booms.
Large electric and hydraulic excavators are typically on crawler tracks and have a large volume bucket that is at the end of a boom and is commonly rated by the cubic yards of material that it will hold. The capacity of most large electric shovels is typically in the range of 70 to 80 cubic yards, commonly quoted as 70 to 80 yards. The capacity of hydraulic excavators is typically in the range of 45 to 50 cubic yards.
Once the operator moves the shovel to the desired area, the boom is swung toward the pile and the bucket is pushed through the pile until it is full of material. In order to maximize the operating time of the shovel, several trucks are used. Trucks typically line up on either side of the large shovel so that, after a truck on one side is loaded, the shovel operator swings to the opposite side to continue operating. A line of trucks is typically formed on each side of the shovel in order to maximize productivity of the shovel and avoid shutting down the loading operation. Operating in this manner, a large electrical shovel with a 70 yard bucket can typically load about 14,000 tons of earth per hour.
Although this production rate is impressive, the efficiency of the shovel is limited by the dead time that occurs with each loading cycle of a truck. A loading cycle includes the time it takes for the operator to drive the bucket through the pile, swing the loaded bucket from the pile while raising it above the truck, then release the load into the truck. The typical cycle time on large shovels is typically around 35 seconds. The truck is therefore sitting idle for much of the time while the operator runs through his loading cycle and this reduces the efficiency of the operation. With the high cost of fuel and the enactment of legislation reducing carbon emissions of trucks, the costs of operating large electric or hydraulic shovels is very expensive. The cost of a large electric shovel is also very expensive, typically in the range of $30 million for a 70 yard shovel.
Accordingly, there is a need to reduce the unproductive time and improve the efficiency in moving large quantities of heavy material to a waiting transport vehicle such as a truck. It would also be beneficial to reduce the cost of the equipment for performing this operation.